1. Field of the Invention
The present invention relates to a photometering device used in exposure control or flash light emission control of a camera.
2. Related Background Art
In recent photometering means of a camera, a technique for determining an optimum exposure amount for an object field by photometering the object field by dividing it into a plurality of areas and processing a plurality of light intensity information, that is, a multi-photometering technique is usually used.
A prior art multi-photometering device is explained with reference to FIGS. 4 to 7.
FIG. 7 shows an example of a division pattern of an object field in the multi-photometering. An object light transmitted through a photographing lens (not shown) is directed to a photo-diode array which is divided into the same pattern H1-H5 as that of FIG. 7 and which is arranged in a camera body, and it is converted to an electrical signal for each of the areas H1-H5.
FIG. 4 shows a block diagram of a prior art photometering circuit.
In FIG. 4, B1 denotes an absolute temperature proportional (hereinafter referred to as T-proportional) voltage generator which generates a T-proportional reference voltage VT which is always proportional to only the absolute temperature.
The T-proportional reference voltage VT is outputted from a T-proportional reference voltage output terminal V.sub.ref through a buffer amplifier A8.
The T-proportional reference voltage VT is divided at a predetermined ratio by a resistor R1 and a resistor R2, and a voltage VK through a buffer amplifier A7 is applied to all cathodes of photo-diodes PD1-PD5.
The photo-diodes PD1-PD5 correspond to the areas H1-H5 of FIG. 7, respectively. The voltages converted by the photo-diodes are proportional to light intensities and they are affected by a temperature. In order to eliminate the effect of the temperature, the T-proportional reference voltage VT is generated to use it as a power supply voltage of a control circuit.
The cathode of the photo-diode PD1 is connected to a non-inverting input terminal of an amplifier A1 and an anode is connected to an inverting input terminal of A1. The inverting input terminal receives a negative feedback from an output terminal of A1 through a diode D1 so that an output voltage VA1 of the amplifier A1 is a logarithmically compressed voltage of a photo-diode current which is proportional to the light intensity. This circuit is called a logarithmic compression circuit. Logarithmic compression circuits for the photo-diodes PD2-PD5 are similarly constructed.
The outputs of the amplifiers A1-A5 are bound into one signal line through analog switches AS1-AS5 and applied to a non-inverting input terminal of an amplifier A6. A constant current source J1 having a constant current I.sub.o as a sink is connected to an inverting input terminal of the amplifier A6, which receives a negative feedback of a diode D6 from an output terminal thereof. The output terminal of the amplifier A6 is connected to a photometering output voltage output terminal VL.
Five channel selection signals S1-S5 outputted from a shift register SR1 control the turn-on and the turn-off of the analog switches AS1-AS5 which are turned on by Low (L) signals.
An input signal R and an input signal CK for controlling the output of the shift register SR1 are applied to input terminals CS and CLK, respectively.
FIG. 5 shows a control unit of a camera which uses the photometering circuit of FIG. 4. The photometering output terminal VL and the T-proportional reference voltage output terminal V.sub.ref of the photometering circuit 5 are connected to AD1 and AD2, respectively, which are input terminals of an analog/digital (A/D) converter built in a microcomputer (CPU) 6. The input terminals CS and CLK of the photometering circuit 5 are connected to output ports P1 and P2 of the CPU 6, respectively. An exposure control circuit 3 and a display circuit 4 receive signals from the CPU 6 to control an iris and a shutter and display the photometering result, respectively.
An operation of FIGS. 4 and 5 is now explained with reference to a timing chart of FIG. 6.
As an initial condition, the CPU 6 sets the outputs of the ports P1 and P2 to High (H) signals. In the photometering circuit 5, when the inputs to the input terminals CS and CLK are both H, only the channel selection signal S1 of the shift register SR1 is L and S2-S5 are H. Accordingly, only the analog switch AS1 of the analog switches AS1-AS5 is turned on and all of AS2-AS5 are turned off. As a result, only the output of the amplifier A1 is connected to the amplifier A6.
A principle of operation of converting a photocurrent to a logarithmically compressed voltage is now explained with reference to the above example.
The photo-diode PD1 produces a photo-current IL1 which is proportional to an input light intensity. Because an input impedance of the amplifier A1 is very high, almost all of the photo-current IL1 flow into D1 and the output VA1 of the amplifier A1 produces a voltage defined as follows. EQU VA1=VK-(kT/q)1n(IL1/I.sub.s) (1)
where
T: absolute temperature PA0 k: Boltzmann constant PA0 q: charge of electron PA0 I.sub.s : backward saturation current of the diode D1.
Since VK is a divided voltage of the T-proportional reference voltage, it is proportional to the absolute temperature. kT/q is also proportional to the absolute temperature. However, since I.sub.s nonlinearly depends on the absolute temperature, the formula (1) is not proportional to the absolute temperature.
When it is applied to the amplifier A6 through the analog switch, the output voltage VA6 of the amplifier A6 is expressed as follows: EQU VA6=VA1+(kT/q)1n(I.sub.o /I.sub.s) (2)
By substituting the formula (1), we get EQU VA6=VK-(kT/q)1n(IL1/I.sub.o) (3)
Thus, the term I.sub.s is erased and the output voltage which is completely proportional to the absolute temperature and which is the logarithmic compression of the light intensity is produced. Thus, the amplifier A6 is called an I.sub.s correction amplifier.
In the initial state, a photometering output voltage VL1 for a light intensity of the area H1 is outputted from the terminal VL. When the CPU 6 causes the output of the port P1 to fall (that is, the input to the input terminal CS of the photometering circuit 5), the shift register SR1 is reset and the output of the clock pulse at the port P2 (the input from the input terminal CLK) is monitored. Under this state, the output of the port P2 of the CPU 6 falls five times at a predetermined time interval. The shift register SR1 then sequentially switches the selection signals S1, S2, . . . S5 to L each time the input from the input terminal CLK falls.
When the first falling edge is applied from the input terminal CLK, the signal S1 is L so that there is no change from the initial state.
When the second falling edge is applied from the input terminal CLK, the signal S2 is only the analog switch AS2 is turned on and the voltage VA2 which is the logarithmically compressed light intensity of the area H2 is connected to the input of the I.sub.s correction amplifier A6, and the photometering output voltage VL2 is outputted from the terminal VL. Similarly, at each fall of the input terminal CLK, the channel selection signals S3, S4 and S5 sequentially assume the L-level and the photometering output voltage from the terminal VL is switched to VL2, VL3, VL4 and VL5. The serial output of the photometering output voltage serves to reduce the number of output terminals. Each time the CPU 6 causes the output of port P1 to fall, it A/D--converts the voltage at the A/D conversion input terminal AD1 and temporarily stores the converted output in a memory. When the A/D conversion and the storing are completed for five areas, it A/D--converts the T-proportional reference voltage which is applied to the A/D conversion input terminal AD2 from the terminal V.sub.ref, and temporarily stores the result.
The CPU 6 calculates the photometering information free from the temperature component based on the A/D conversion outputs of the five areas and the A/D conversion output of the T-proportional reference voltage, which are stored in the memory, and calculates control data for the exposure control and the display of the photometering results based on the calculated photometering information and an exposure amount calculation algorithm, and supplies signals to the exposure control circuit 3 and the display circuit 4.
In the prior art, the light intensity distribution information for the multi-photometering is obtained by the construction and operation described above.
In the prior art, one output terminal for outputting an analog voltage which is generated in accordance with the light intensity of the object field and one output terminal for outputting the T-proportional reference voltage are required to eliminate the temperature component. On the other hand, it is a common technique to apply the analog voltage and the T-proportional reference voltage to the A/D converter of the CPU 6, convert them to the digital signals, and calculate a ratio of those signals by the arithmetic operation means to obtain the photometering output.
Thus, two of the limited number of A/D converters of the CPU 6 are occupied, and a problem of shortage of A/D converters occurs when the A/D converters are to be used for other purposes such as CCD data for range finding.